J. Phys. Chem. A 2005, 109, 5481-5491
5481
Electronic Structure, Excited States, and Photoelectron Spectra of Uranium, Thorium, and Zirconium Bis(Ketimido) Complexes (C5R5)2M[-NCPh2]2 (M ) Th, U, Zr; R ) H, CH3) Aurora E. Clark,* Richard L. Martin, and P. Jeffrey Hay Theoretical DiVision, Los Alamos National Laboratory, Mail Stop B268, Los Alamos, New Mexico 87545
Jennifer C. Green Inorganic Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford, OX1 3QR, United Kingdom
Kimberly C. Jantunen and Jaqueline L. Kiplinger Chemistry DiVision, Los Alamos National Laboratory, Mail Stop J514, Los Alamos, New Mexico 87545 ReceiVed: January 19, 2005; In Final Form: April 18, 2005
Organometallic actinide bis(ketimide) complexes (C5Me5)2An[-NdC(Ph)(R)]2 (where R ) Ph, Me, and CH2Ph) of thorium(IV) and uranium(IV) have recently been synthesized that exhibit chemical, structural, and spectroscopic (UV-Visible, resonance-enhanced Raman) evidence for unusual actinide-ligand bonding. [Da Re et al., J. Am. Chem. Soc., 2005, 127, 682; Jantunen et al., Organometallics, 2004, 23, 4682; Morris et al., Organometallics, 2004, 23, 5142.] Similar evidence has been observed for the group 4 analogue (C5H5)2Zr[-NdCPh2]2. [Da Re et al., J. Am. Chem. Soc., 2005, 127, 682.] These compounds have important implications for the development of new heavy-element systems that possess novel electronic and magnetic properties. Here, we have investigated M-ketimido bonding (M ) Th, U, Zr), as well as the spectroscopic properties of the highly colored bis-ketimido complexes, using density functional theory (DFT). Photoelectron spectroscopy (PES) has been used to experimentally elucidate the ground-state electronic structure of the thorium and uranium systems. Careful examination of the ground-state electronic structure, as well as a detailed modeling of the photoelectron spectra, reveals similar bonding interactions between the thorium and uranium compounds. Using time-dependent DFT (TDDFT), we have assigned the bands in the previously reported UV-Visible spectra for (C5Me5)2Th[-NdCPh2]2, (C5Me5)2U[-NdCPh2]2, and (C5H5)2Zr[-NdCPh2]2. The low-energy transitions are attributed to ligand-localized N p f CdN π* excitations. These excited states may be either localized on a single ketimido unit or may be of the ligand-ligand charge-transfer type. Higherenergy transitions are cyclopentadienyl π f CN π* or cyclopentadienyl π f phenyl π* in character. The lowest-energy excitation in the (C5Me5)2U[-NdPh2]2 compound is attributed to f-f and metal-ligand chargetransfer transitions that are not available in the thorium and zirconium analogues. Geometry optimization and vibrational analysis of the lowest-energy triplet state of the zirconium and thorium compounds also aids in the assignment and understanding of the resonance-enhanced Raman data that has recently been reported. [Da Re et al., J. Am. Chem. Soc., 2005, 127, 682.]
Introduction The chemistry of bent metallocene complexes has been extensively studied with organometallic group 4 elements (C5H5)2ML2 (where M ) Ti, Zr, Hf; L ) halogen)1,2 as well as with organoactinide complexes (C5Me5)2AnL2 (where An ) Th, U; L ) Ph, CH2Ph, CH3).3,4 The bonding and electronic properties have also been investigated with a variety of techniques, such as ultraviolet-visible (UV-Vis) spectroscopy, cyclic voltammetry, and photoelectron spectroscopy (PES).1-3,5,6 Recently, organoactinide species (C5Me5)2AnL2 of thorium(IV) and uranium(IV) that involve organic ketimido [L ) -NdC(R)(R′)] ligands have been synthesized.3,4 In particular, the ketimido complexes exhibit structural and spectroscopic evidence for an actinide-ligand bond order that may be slightly greater than one. This has important implications for the development of new actinide systems that possess novel * To whom correspondence should be addressed:
[email protected].
electronic and magnetic properties, where the electronic structure of the metal can couple intimately with the ligand. The electronic properties of group 4 metallocene complexes such as (C5H5)2MCl2 (where M ) Ti, Zr, Hf) have been probed using absorption and electronic luminescence spectroscopies.1,2 These metallocenes are formally d0 complexes, and their excited states are ligand-to-metal charge-transfer (LMCT) in character. Excitations can arise both from orbitals on the halogen (Cl f M) and the cyclopentadienyl (C5H5 f M), and assignments of the photoelectron and absorption spectra show both types of charge-transfer excitations to be close in energy.1,2 Recent density functional theory (DFT) calculations have provided more insight into the nature of the excitations, in which the character of the highest occupied molecular orbital (HOMO) changed from equal mixtures of Cl and C5H5 to being more cyclopentadienyl-localized, as one goes from Ti to Zr to Hf.2 More recently, the emission from (C5H5)2Ti(NCS)2 was studied and intense phosphorescence from a LMCT triplet excited state was
10.1021/jp050339d CCC: $30.25 © 2005 American Chemical Society Published on Web 05/28/2005
5482 J. Phys. Chem. A, Vol. 109, No. 24, 2005 observed at 77 K.7 DFT analysis ascribed this excited state to a π(NCS) f d(Ti) excitation. Experimental investigations using PES have probed the bonding of thorium and uranium metallocene and related complexes, usually coupled with DFT or other theoretical methods. Recent examples of these complexes include (C5H5)2M(CH2SiMeCH2) (M ) Ti, Zr, Mo, Th),8 (C5H5)3AnL (An ) U, Th; L ) CH3, BH4),9 and An[η8-C8H4(1,4-SiiPr3)2]2 (An ) Th, U).10 Earlier studies of the photoelectron spectra of (C5Me5)2MX2 (M ) U, Th, Zr; X ) CH3, Cl) complexes have also utilized DFT to interpret bonding.11,12 Following the synthesis of the uranium bis(ketimido) complex (C5Me5)2U[-NdCPh2]2 by Kiplinger et al.,4 additional structures have been reported for thorium and uranium complexes of the type (C5Me5)2An[-NdCPh(R)]2, where R ) Ph, Me, and CH2Ph.3 The structural details of these complexes will be discussed later in this paper, in comparison to the theoretical studies. Although most thorium(IV) complexes with the 6d05f0 configuration are colorless, the thorium bis(ketimido) complexes are orange, possessing weak absorption in the visible region of 450-550 nm (18 000-22 000 cm-1) and strong absorption bands that extend into the UV region up to 300 nm (33 000 cm-1).5 The red-colored uranium(IV) bis(ketimido) complexes have similar intense broad absorption bands, with weaker bands extending to even lower energy regions in the visible range of 500-700 nm (14 000-20 000 cm-1).5 Previous studies by Morris et al.13 have examined the spectroscopy and electrochemistry of uranium(IV) and thorium(IV) compounds. There, the (C5Me5)2AnL2 species (where L ) Cl, SO3CF3, CH3, CH2Ph, imido (dN-2,4,6-tBu3C6H2), hydrazonato (η2-(N,N′)-R-N-NdCPh2 (R ) CH3, CH2Ph, and Ph), and ketimido (-NdC(Ph)(R); R ) CH3, CH2Ph)) are categorized as Class I, II, and III, respectively, according to increasing molar absorptivity. Ligands in the Class I complexes include σ-donors such as Me, whereas Class II and Class III species are composed of complexes that contain nitrogen-containing ligands that have the capability to interact in both the σ- and π-fashion with the metal. Thus, the complexes in Classes II and III can have multiple bonding character between the An-N and include the hydrazonato (Class II), ketimido, and imido (Class III) actinide compounds. The resonance Raman spectroscopy of An(IV) ketimido complexes was probed in more detail by Da Re et al.5 The visible near-infrared (Vis-NIR) absorption region for these complexes was deconvoluted into four absorption bands, denoted as bands I-IV. Band I occurs at higher energy (2022 000 cm-1), whereas bands II-IV extend to lower energy (12 000-20 000 cm-1). Raman bands showing enhancement in symmetric and asymmetric CdN stretching modes are observed upon excitation in band I. This evidence was used to assign band I as arising from ligand-localized (ketimido) excitation. Additional spectroscopic studies of (C5Me5)2Th[-NdCPh2]2 and (C5H5)2Zr[-NdCPh2]2 were performed, facilitating the assignment of bands II-IV to metal-to-ligand charge-transfer excitations. Although the spectral (UV-Vis and resonance-enhanced Raman) and electrochemical properties of 5f 2 uranium(IV) complexes and the analogous f0d0 thorium(IV) complexes have been studied,5,13 there have been no theoretical calculations to elucidate and understand their molecular properties. In this work, theoretical and spectroscopic techniques are used to explore the ground- and excited-state electronic structure of thorium, uranium, and zirconium ketimide complexes. PES is used to probe the ground-state bonding properties of the thorium and uranium ketimide compounds, whereas theoretical calculations
Clark et al. present a detailed analysis of the recently reported vibrational and UV-Vis spectra of the thorium, uranium, and zirconium complexes.5 The theoretical calculations utilize DFT to model the photoelectron spectra and describe the ground-state properties, whereas time-dependent DFT (TDDFT) provides a platform to understand and discuss the excited states. Computational Methods The calculations presented for (C5H5)2Th[-NdCPh2]2 (1), (C5H5)2Th[-NdCH2]2 (1a), (C5H5)2Zr[-NdCPh2]2 (2), and (C5H5)2U[-NdCPh2]2 (3) used the B3LYP hybrid density functional approach,14,15 as implemented in the Gaussian03 program.16 The LANL2 effective core potential17 (ECP) was used to replace the inner 28 electrons of Zr, whereas the Stuttgart relativistic electron effective core potential18 (RECP) was used to replace the inner 60 electrons of Th and U. The outer core 4s, 4p and valence 5s, 4d, and 5p electrons of Zr were treated with the uncontracted LANL2DZ basis,17 whereas the inner core 5s, 5p, the outer core 5d, 6s, 6p electrons and valence 5f, 6d, 7s, and 7p electrons of U and Th were treated using a [6s 6p 5d 3f] contracted Gaussian basis. The 6-31G basis19 was used to treat the C, N, and H atoms, respectively. The geometries of 1-3 were optimized and their vibrational frequencies calculated using analytic second derivatives. The structures were verified to be global minima with no imaginary vibrations. The geometries of the first triplet excited states of 1 and 2 were also optimized and confirmed to be global minima. The photoelectron spectra of 1 and 3 were modeled to firstorder by the method of Gelius,20 in which the one-electron molecular orbital cross section is approximated by
σMO )
cAO,MOσAO ∑ AO
where σAO is a theoretical atomic orbital cross-section value at an energy appropriate for the incident photon source.21 The coefficient cAO,MO describes the extent to which each atomic orbital contributes to the molecular orbital. This method qualitatively models the effect of orbital delocalization on the intensities.22 Lichtenberger and colleagues have demonstrated that this approach reasonably describes the cross sections associated with the low-kinetic-energy electrons obtained from sources such as helium(I) and helium(II).23 Because the thorium ketimido molecular orbitals (MOs) exhibit significant f atomic orbital (AO) character and the Th formally has a 7s26p66d2 atomic electron configuration, no atomic f AO cross section is available for Th. As such, we used the f cross-section value of U as an approximation. The predicted MO cross-section values were broadened with Gaussian functions with a width of 0.1 eV. The relative energies of all the MOs were then shifted such that the ionization energy of the HOMO matched the lowestenergy ionization peak in the PES. This amounted to a shift of 2.1 and 0.8 eV in the energies of the thorium and uranium ketimido MOs, respectively. Because unrestricted calculations were used on the uranium(IV) compound, the cross-sectional contribution from the R and β orbitals was averaged for the simulation. The excited electronic states were determined from TDDFT,24-28 which was performed at the optimized groundstate geometry, using the same functionals, core potentials, and bases. The natural transition orbitals (NTOs) for a given excited state were determined using the formulation of Martin29,30 that was used in the development version of Gaussian.
U, Th, and Zr Bis(ketimido) Complexes
Figure 1. (U)B3LYP optimized geometries of (C5H5)2Th[-NdCPh2]2 (1), (C5H5)2Zr[-NdCPh2]2 (2), and (C5H5)2U[-NdCPh2]2 (3) complexes. Cyclopentadienyl ligands are not shown for clarity. Experimental quantities are given in parentheses, bond lengths are given in angstroms, and bond angles are given in degrees.
Experimental Methods The compounds (C5Me5)2Th[-NdCPh2]2 (1*) and (C5Me5)2U[-NdCPh2]2 (3*) were synthesized as previously reported.3,4 The helium(I) and helium(II) photoelectron spectra of 1* and 3* were obtained using a Helectros model 0078 PE spectrometer that was fitted with a hollow cathode discharge lamp. Spectra were acquired by repeat scanning, using an Atari microprocessor, and calibrated using N2, Xe, and He, similar to previous reports.22 Results and Discussion 1. Ground-State Electronic Structure of (C5H5)2M[-Nd CPh2]2 Complexes (M ) U, Th, Zr). 1.1. Geometry. The optimized ground-state structures for (C5H5)2Th[-NdCPh2]2 (1), (C5H5)2Zr[-NdCPh2]2 (2), and (C5H5)2U[-NdCPh2]2 (3) are presented in Figure 1. The structural parameters are quite similar to the X-ray crystal structure values of the permethylated analogues 1* and 3*, as well as 2 with bond lengths deviating by (0.03 Å and bond angles deviating by
